Electronic Transitions in
Perovskite
Possible Nonconvecting Layers in
the Lower Mantle
by
James Badro et al.
Heat Transport
„
„
„
Conduction, Radiation or Convection
Convection only occurs if the former
two methods fail to transfer heat
Changes in mineral conduction and
radiation properties strongly affect
mantle dynamics
Mantle Composition
„
„
80% perovskite
(Mg,Fe)O undergoes a high-spin to
low-spin transition between 60-70
GPa
Blue-shift
„
„
„
Iron absorption bands shift to blue
from IR and Red at high-pressure
Badro et al. report two such shifts at
70 and 120 GPa
This indicates to two regions of high
IR and red transparency in the
mantle…
K β-emission spectroscopy
„
„
„
„
Originates from 3pÆ1s decay
Interaction of X-rays with K-shell,
the inner electron shell
Characteristic peaks for high-spin
and low-spin electrons
HS and LS are properties defined by
the interaction of 3p and 3d shells
Image removed due to copyright considerations.
(Mg,Fe)-perovskite
Site and Spin
„
„
„
„
„
„
„
56% ferrous in dodecahedral (2+)
25% ferric in octahedral (3+)
19% ferrous in octahedral (2+)
So 56% (ferrous) and 44% (ferric)
Mixed state (HS and LS)
55%HS, 45% LS
Don’t let the numbers fool you
But an interesting point is introduced
Image removed due to copyright considerations.
(Mg,Fe)-perovskite
Further Quantification Needed
„
Of Al affect on compressibility
„
Temperature effect (15-18 GPa?)
Post-perovskite
„
„
“Conjecture” a connection with the
HS-LS transition
Propose that further theoretical
modeling of mixed spin states of
(Mg,Fe)O and (Mg,Fe)pv are needed
Light Absorption
„
„
„
Radiative conductivity is hindered by
IR light absorption in HS Fe-bearing
lower-mantle phases
In the LS state, IR absorption is
hindered
Indeed, even laser heating did not
work above 120 GPa!
Image removed due to copyright considerations.
The figure below shows the normalized blackbody thermal radiation as a function of wavelength at three different
temperatures (2000, 2500, and 3000 K) that can be considered as bounds for the lower-mantle geotherm (S7).
The absorption bands of (Mg0.9 Fe0.1)SiO3 perovskite are reported in the HS state (orange sticks) according to
pressure-corrected measurements of Keppler et al. (S8) performed at room conditions, and in the LS state (blue
sticks) according to the model proposed by Burns (S9). In the HS state, the most intense absorption bands are in
the near infrared (IR) region, where blackbody radiation is maximal at these temperatures (S5); this makes
perovskite a very bad radiative thermal conductor. In the LS state, these same bands undergo a blue-shift to the
visible region, where blackbody radiation is weaker. The total power throughput in perovskite therefore dramatically
increases after the transition, since IR radiation travels with a larger mean free path. This contributes to an
enhanced radiative thermal conductivity in this state (S5), and is characteristic of the mineral assemblages (see
text) of Earth’s lowermost mantle.
Correlation with the Mantle
„
„
„
„
70 GPa transition consistent with
1700 km depth consistent with
transition pressure for Fe in
(Mg,Fe)O
120 GPa consistent with D” layer
Increased radiative conductivity
Rayleigh number is decreased,
hindering convection, favoring layers